A piercing machine is used for the production of seamless steel pipes in the Mannesmann process. A piercing machine includes a pair of skew rolls and a plug. The plug is disposed between the pair of skew rolls and is on a pass line. The piercing machine pushes and squeezes a billet over the plug while rotating it in the circumferential direction by means of the skew rolls to piercing-roll the billet into a hollow blank.
The piercing machine piercing-rolls a heated billet. As a result, the plug over which the billet is squeezed is exposed to high temperature and is subjected to high pressure. Therefore, the plug is susceptible to melting loss and scoring.
Generally, an oxide scale is formed on the surface of the base metal of the plug. The oxide scale blocks the heat from the billet thereby suppressing the occurrence of melting loss. The oxide scale further suppresses the occurrence of scoring.
However, the oxide scale will wear every time the plug piecing-rolls a billet. When the oxide scale is lost, the temperature of the base metal of the plug will rise, resulting in a melting loss of the plug.
To improve the life (number of uses) of the plug, forming a coating other than an oxide scale on the surface of the base metal of the plug is proposed.
JP4279350B discloses the forming of a sprayed film made up of oxides and Fe by arc spraying an iron wire rod on the surface of the base metal of the plug.
Further, JP2776266B, JP3891679B, and JP2009-101408A disclose the forming of a build-up layer on the surface of the base metal of the plug.